Contemporary engine misfire detection systems for reciprocating engines apply misfire detection to determine improper combustion. Misfiring causes a loss of power, dumps unburned fuel into a catalytic converter-thereby shortening its life, and causes higher levels of hazardous emissions. The need to identify whether a misfire has occurred is a strict requirement of current government regulation.
One approach of current misfire detection systems is to use a measure of a deviation away from an expected crankshaft velocity or acceleration. Many of these systems are effective but needlessly complex. To meet the regulations these systems must process engine crankshaft position data at very high data rates. This requirement puts a significant demand on associated signal processing circuitry which makes it complex. Furthermore, to effectively implement this approach, the engine crankshaft position must be sensed at a fairly high resolution. This requires a significant investment in tooling of mechanical engine parts to enable measurement of engine crankshaft position at high resolutions.
Another scheme monitors deviations in exhaust pressure and analyzes the resulting harmonic spectra from an engine via the engine's exhaust system. Sensor durability is a major factor in these systems because of the vexed hostile environment in the exhaust system. Furthermore, this scheme's accuracy is substantially dependent on the characteristics of the coupling medium, in this case the exhaust system. The exhaust system, includes an exhaust manifold, coupled to an exhaust tube that is coupled to a catalytic converter, that is coupled to a muffler, that is coupled to an exhaust pipe. Because of this structure, this arrangement is susceptible to interference from non-engine performance related audio noise sources including engine and vehicle vibrations that are coupled into the exhaust system. A resonance of this coupling medium may add to the harmonic spectra provided by the engine. Also, because of its large volumetric size, the exhaust system acts like a lowpass filter that reduces the available signal thus effecting the accuracy of the measurement. Additionally, the propagation time of audio output from the engine will change as the exhaust system heats up or cools down. Also, the length that the individual cylinder audio output traverses varies with each cylinder because of the different exhaust runner lengths of the exhaust manifold. This will cause a variable delay from when the exhaust valve opens to when it is sensed. This variable length coupling from each cylinder to the sensory means may also shift the harmonic spectra provided by the engine. This is because of the pressure wave reflections that are caused by the different amount of time a pressure pulse will take to travel from an exhaust valve to the audio sensor in different cylinders.
What is needed is an improved approach for misfire detection for engines with relatively low complexity that is insensitive to inaccuracies of the prior art approaches.